1. Field of the Invention
The present invention relates to a technology of detecting dirt on a linear scale in an image forming apparatus.
2. Description of the Related Art
Various types of image forming apparatuses, such as a printer, facsimile, copier, plotter, and a multifunction product combining functions of printing, faxing, and copying, include a serial-type image forming apparatus and a line type image forming apparatus. In the serial-type image forming apparatus, a carriage carrying a recording head including liquid ejection heads for ejecting droplets of a liquid (hereinafter “ink”) is caused to scan serially in a direction perpendicular to a conveying direction of a record-bearing medium (hereinafter “sheet”, which can be made of paper or other nonpaper materials, and can also called “recording medium”, “recording paper”, or “printing material”) as the record-bearing medium is conveyed intermittently according to a recording width, so that conveying and recording is repeated alternately to form an image (i.e., to record and print characters and images, etc.) on the record-bearing medium. In the line-type image forming apparatus, on the other hand, recording is carried out while conveying the record-bearing medium without moving the recording head.
For example, for the serial-type image forming apparatus, to form a high quality image by causing the recording head to eject recording liquid droplets to given spots on the sheet, it is important to ensure the constant speed of the carriage, which is a moving member carrying the recording head, and the sheet-feeding precision of a conveying unit. Generally, a direct current (DC) motor is used as a driving source for moving the carriage and conveying unit, which are driven under servo control.
In detecting the position of the carriage through DC servomotor control, however, if a linear scale (also called as encoder sheet), which composes a linear encoder for obtaining position/speed information, becomes dirty with attachment, such as ink, and grease or oil applied to a guide rod for guiding the movement of the carriage, a detection signal from the linear encoder (reading signal from the linear scale) becomes inaccurate to make it difficult to accurately detect the position of the carriage.
A technology is disclosed in Japanese Patent Application Laid-Open No. 2002-225374, in which a mode for detecting dirt on a linear scale is provided to count the number of slits formed on the linear scale for detecting the presence/absence of the dirt. When it is detected that the dirt is present on the linear scale, the position of the dirt is calculated.
Furthermore, an image forming apparatus is disclosed in Japanese Patent Application Laid-Open No. 2006-007441, which does not detect dirt on a linear scale but detects the speed or position of a moving body based on a reading signal output from an encoder in response to the displacement of the moving body, and which controls the displacement of the moving body based on a result of the detection. The image forming apparatus is provided with a unit that determines whether the reading signal is normal based on the signal width of the reading signal and a preset signal width while the moving body is displaced at a constant speed.
According to the configuration described in Japanese Patent Application Laid-Open No. 2002-225374, the number of slits formed on the linear scale is counted to detect the presence/absence of the dirt in the dirt detection mode, and the position of the dirt is detected through calculating on the counted number. Such configuration, therefore, has problems that a specific mode for detecting the dirt is necessary and that easily grasping the position of the dirt is difficult.